In recent years, lithium ion secondary batteries which charge and discharge by the transfer of lithium ions between negative and positive electrodes have been intensively studied and developed as high-energy density batteries.
Also in recent years, metal complex oxides have been noted as lithium hosts of negative electrodes. In particular, titanium oxides allow quick and stable charging and discharging because of their potential characteristics. In addition, negative electrodes containing titanium oxide as an active material have a longer life than carbon materials. However, the titanium oxides have a higher potential for metal lithium than do common carbon negative electrodes and have a lower capacity density per unit weight. Therefore, their energy densities, which are an important factor for secondary batteries, are low.
On the other hand, the electrode potential of titanium oxide is caused by the oxidation-reduction reaction between Ti3+ and Ti4+ during electrochemical insertion and extraction of lithium, and a potential of about 1.5V is generated with reference to metal lithium. When lithium is inserted and extracted through the use of oxidation and reduction of titanium, the electrode potential is electrochemically limited, so that it is substantially difficult to decrease the electrode potential for the purpose of increasing the energy density. Accordingly, the maximization of the theoretical electrode capacity of the titanium oxide is very important for increasing the energy density. In general, titanium oxide containing no lithium ion has poor electron conductivity, and thus must be combined with conductive additives when used as an active material. On the other hand, in order to allow smooth transfer of lithium ions in titanium oxide, atomization and improvement of the crystal form are necessary.
Under the above circumstances, there is a known technique for improving the contact area between a conductive agent and an active material; the surface of an active material is coated with a conductive agent, thereby establishing a surface contact between them. The electrode will have improved electron conductivity, but the conductive agent located on the surface of the active material, or in the channel of insertion and extraction of lithium ions hinders the transfer of lithium ions, and thus deteriorates the ion conductivity.